Chemical and isotopic evidence for sources of fluids in a mud volcano field seaward of the Barbados accretionary wedge

Chemical, isotopic, and heat budgets are controlled by fluid flow and venting at convergent margins. Fluids vent from accretionary complex sediments via numerous pathways including mud volcanoes, which are common at the Barbados convergent margin. Chemical and isotopic compositions of fluids collected from four mud volcanoes, located east of the Barbados deformation front at 13 degrees 50'N, indicate venting is sufficiently rapid to prevent diffusion from seawater. In sediments surrounding the mud volcanoes, the rate of nonfocused fluid flow is similar to 0.26 cm/yr. Although this rate is similar to 3 orders of magnitude slower than the flow from the mud volcanoes, significantly more fluid vents from the sediments surrounding the mud volcanoes than from the restricted areas of rapid flow from the volcanoes. Chloride concentrations are depleted to a minimum of 211 mM (similar to 45% of seawater value) primarily by mixing diagenetically altered pore fluids with an O-18-enriched fresh water released from the dissociation of methane hydrate. This reaction is accompanied by the release of sufficient methane to form a free gas phase and initiate eruption of the mud volcanoes. The geochemical compositions of the diagenetically altered pore fluids reflect the interplay between five major reactions: (1) clay mineral dehydration, which releases H2O and influences a range of chemical and isotopic compositions; (2) organic matter regeneration, which increases alkalinity, NH4+, and Br- and decreases SO42- concentrations; (3) cation exchange with clay minerals, which increases Na+ and decreases NH4+ concentrations; (4) carbonate recrystallization and precipitation, which decrease Ca2+ but increase Sr2+ concentrations; and (5) conversion of volcanic ash to clay minerals, which utilizes H2O and decreases Mg2+ concentrations. The geochemistry of the pore fluids suggests that these reactions occurred at temperatures ranging from similar to 75 degrees to 115 degrees C. Thus at the regional geothermal gradients of similar to 27 to 40 degrees C/km, these temperatures indicate that the source regions are at similar to 2 to 4.5 km depth.